The term “oil shale” refers to a sedimentary rock interspersed with an organic mixture of complex chemical compounds collectively referred to as “kerogen.” The oil shale consists of laminated sedimentary rock containing mainly clay with fine sand, calcite, dolomite, and iron compounds. Oil shales can vary in their mineral and chemical composition. When the oil shale is heated to above 250-400° F., destructive distillation of the kerogen occurs to produce products in the form of oil, gas, and residual carbon. The hydrocarbonaceous products resulting from the destructive distillation of the kerogen have uses which are similar to petroleum products. Indeed, oil shale is considered to be one of the primary sources for producing liquid fuels and natural gas to supplement and augment those fuels currently produced from petroleum sources.
Processes for recovering hydrocarbonaceous products from oil shale may generally be divided into in situ processes and above-ground processes. In situ processes involve treating oil shale which is still in the ground in order to remove the hydrocarbonaceous products, while above-ground processes require removing the oil shale from the ground through mining procedures and then subsequently retorting in above-ground equipment. Clearly, in situ processes are economically desirable since removal of the oil shale from the ground is often expensive. However, in situ processes are generally not as efficient as above-ground processes in terms of total product recovery.
Historically, prior art in situ processes have generally only been concerned with recovering products from oil shale which comes to the surface of the ground; thus, prior art processes have typically not been capable of recovering products from oil shale located at great depths below the ground surface. For example, typical prior art in situ processes generally only treat oil shale which is 300 feet or less below the ground surface. However, many oil shale deposits extend far beyond the 300 foot depth level; in fact, oil shale deposits of 3000 feet or more deep are not uncommon.
Moreover, many, if not most, prior art processes are directed towards recovering products from what is known as the “mahogany” layer of the oil shale. The mahogany layer is the richest zone of the oil shale bed, having a Fischer assay of about twenty-five gallons per ton (25 gal/ton) or greater. The Mahogany Zone in the Piceance Creek Basin consists of kerogen-rich strata and averages 100 to 200 ft thick. This layer has often been the only portion of the oil shale bed to which many prior art processes have been applied.
For economic reasons, it has been found generally uneconomical in the prior art to recover products from any other area of the oil shale bed than the mahogany zone.
Thus, there exists a relatively untapped resource of oil shale, especially deep-lying oil shale and oil shale outside of the mahogany zone, which have not been treated by prior art processes mainly due to the absence of an economically viable method for recovering products from such oil shale.
Another important disadvantage of many, if not most prior art in situ oil shale processes is that expensive rubilization procedures are often necessary before treating the oil shale. Rubilization of the in situ oil shale formation is typically accomplished by triggering underground explosions so as to break up the oil shale formation. In such prior art process, it has been necessary to rubilize the oil shale formation so as to provide a substantial reduction in the particle size of the oil shale. By reducing the particle size, the surface area of the oil shale treated is increased, in an attempt to recover a more substantial portion of products therefrom. However, rubilization procedures are expensive, time-consuming, and often cause the ground surface to recede so as to significantly destroy the structural integrity of the underground formation and the terrain supported thereby. This destruction of the structural integrity of the ground and surrounding terrain is a source of great environmental concern.
Rubilization of the oil shale in prior art in situ processes has a further disadvantage. Upon destructive distillation of the kerogen in the oil shale to produce various hydrocarbonaceous products, these products seek the path of lease resistance when escaping through the marlstone of the oil shale formation. By rubilizing the oil shale formation, many different paths of escape are created for the products; the result is that it is difficult to predict the path which the products will follow. This, of course, is important in terms of withdrawing the products from the rubilized oil shale formation so as to enable maximum recovery of the products. Since the products have numerous possible escape paths to follow within the rubilized oil shale formation, the task of recovering the products is greatly complicated.
Oil/tar sands, often referred to as ‘extra heavy oil,’ are types of bitumen deposits. The deposits are naturally occurring mixtures of sand or clay, water and an extremely dense and viscous form of petroleum called bitumen. They are found in large amounts in many countries throughout the world, but are found in extremely large quantities in Canada and Venezuela.
Due to the fact that extra-heavy oil and bitumen flow very slowly, if at all, toward producing wells under normal reservoir conditions, the sands are often extracted by strip mining or the oil made to flow into wells by in situ techniques which reduce the viscosity by injecting steam, solvents, and/or hot air into the sands. These processes can use more water and require larger amounts of energy than conventional oil extraction, although many conventional oil fields also require large amounts of water and energy to achieve good rates of production.
Like all mining and non-renewable resource development projects, oil shale and sands operations have an effect on the environment. Oil sands projects may affect the land when the bitumen is initially mined and with large deposits of toxic chemicals, the water during the separation process and through the drainage of rivers, and the air due to the release of carbon dioxide and other emissions, as well as deforestation. Clearly any improvements in the techniques use to extract hydrocarbonaceous products from shale and sands would be appreciated, particularly if efficiency is improved and/or environmental impact is reduced.
Certain improvements with respect to the recovery of products from shale are disclosed in U.S. Pat. No. 7,041,051. Unlike other prior art processes, the in situ body of oil shale to be treated is not rubilized. Rather, the process includes drilling a hole in the body of nonrubilized oil shale, and locating a processing gas inlet conduit within the hole such that the bottom end of the processing inlet gas conduit is near the bottom of the hole. An effluent gas conduit is anchored around the opening of the hole at the ground surface of the body of oil shale. A processing gas is introduced into an above-ground combustor. In the combustor, the processing gas, which contains enough oxygen to support combustion, is heated by burning a combustible material introduced into the combustor in the presence of the processing gas. The resultant heated processing gas is of a temperature sufficient to convert kerogen in the oil shale to gaseous hydrocarbonaceous products.
The heat from the heated processing gas, as well as radiant heat from the processing gas inlet conduit, create a nonburning thermal energy front in the oil shale surrounding the hole. The kerogen is thus pyrolyzed and converted into hydrocarbonaceous products. The products produced during pyrolysis of the kerogen are in gaseous form and are withdrawn with the processing gas as an effluent gas through the hole and into the effluent gas conduit. The effluent gas is transferred through the effluent gas conduit into a condenser where the effluent gas is allowed to expand and cool so as to condense a portion of the hydrocarbonaceous products into a liquid fractions. In the condenser, a remaining gaseous fraction of hydrocarbonaceous products is separated from the liquid fraction of hydrocarbonaceous products. The gaseous fraction is preferably filtered and or scrubbed so as to separate the upgraded gas products from any waste gases including the inorganic gas carbon dioxide.
According to the '051 Patent, expensive and time-consuming rubilization procedures are eliminated, and the structural integrity of the ground and surrounding terrain are preserved. While a portion of the upgraded hydrocarbon gas may be recycled to the combustor to provide combustible material for fueling combustion within the combustor, and while a portion of the waste inorganic gas may be recycled to the compressor for augmenting the supply of carbon dioxide in the processing gas, further improvements are possible, both in the generation of the heated, processing gas as well as the recovery of products and byproducts produced in the condenser.